Hybrid and electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric motor which is driven by a power converter with a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain. Traditional HEVs implement two three-phase pulse width modulated (PWM) inverter modules and two three-phase AC machines (e.g., AC motors) each being driven by a corresponding one of the three-phase PWM inverter modules that it is coupled to.
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable frequency drives to control the torque applied to the shaft (and thus finally the speed) of a three-phase AC electric motor by controlling the current fed to the three-phase AC electric motor. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the three-phase AC electric motor. This technique requires knowledge of the rotor's angular position (i.e., the mechanical rotational angular position of rotor relative to the “stator” or motor windings).
The rotor's angular position can be computed based on actual measured quantities using some type of speed or position sensor for control feedback measurement. For instance, to determine the angular position of the rotor, its angular speed can be measured with a speed sensor, and the angular position can then be obtained by integrating the speed measurements. Other systems may use a resolver and resolver-to-digital converter circuit which provides absolute position information directly. A high performance drive system requires a speed or position sensor which is an expensive component. Moreover, the circuitry required to process its signals can also be expensive. The presence of the speed/position sensor in the system adds cost, size, and weight, and reduces reliability as well. It would be desirable to eliminate this speed/position sensor and replace the measured quantities by computed estimates. It would also be desirable to eliminate mechanical interface hardware, reduce cost and weight, and improve the reliability of an electric traction drive system
According to one approach for eliminating the sensor, the rotor's angular position can also be estimated without actually using a sensor to measure actual speed/position quantities. In this regard, there are numerous methods for estimation of the rotor's angular position that are suitable for zero/low speed operation.
Many common sensorless position control methods of a traction drive systems either rely on spatial variation of rotor saliency of a rotor of the drive system or back EMF of the inherent saliency machine of the drive system. These methods are more suitable with Interior Permanent Magnet Synchronous Motor (IPMSM), Synchronous Reluctance Motor and Switched Reluctance Motor machine types which inherently have magnetically salient rotors.
Other methods of detecting rotor angular position include high frequency signal injection and modified PWM test pulse excitation.
In the high frequency signal injection method, a balanced high frequency test signal, such as a voltage (or current) signal, can be injected on a stator winding of an inherently salient machine and the resultant effect of the balanced high frequency test signal on stator current (or voltage) can be measured. The effect of the balanced high frequency test signal injection can be observed in a measured stator current which takes the form of amplitude modulation at two times the fundamental frequency rate. This effect is due to the spatial modulation of the magnetic saliency as the rotor rotates. This method works quite well when the machine under test has inherent saliency, such as an Interior Permanent Magnet type machine. However, Surface Mount Permanent Magnet (SMPM) machines have no intentionally designed saliency and therefore require a very high magnitude injection signal in order to retrieve the position information. Thus, due to additional losses and noise generated by such a high magnitude injection signal, this method is not suitable for SMPM type application.
In the modified PWM test pulse excitation method, modified PWM test pulses can be used to excite the high frequency impedance of the machine. Modified PWM test pulses excite two types of saliencies: 1) mechanical saliency and 2) electrical saliency. When PWM test pulses are injected, the current control is ignored for the test period. This can be a good method for an industrial drive. However, a traction machine has low inductance and not controlling current during test period may result in an uncontrolled condition. This technique retrieves the position information from sensed stator current which must be sampled immediately after injecting the test pulses. This increases number of times the stator current is being sampled.
For example, such techniques have been described for use with induction motors in the following publications: “Sensorless position control of induction motors—an emerging technology,” by Dr. J. Holtz, IEEE Trans. Ind. Electron., vol. 45, pp. 840-852, December 1998, and “Elimination of saturation effects in sensorless position controlled induction motors,” by Dr. J. Holtz and H. Pan, Conf. Rec. IEEE-IAS Annu. Meeting, Pittsburgh, Pa., vol. 3, Oct. 13-18, 2002, pp. 1695-1702. These techniques modify standard PWM waveforms to excite each phase of the machine in turn such that an estimate of the rotor's angular position can be obtained. The technique has been shown to perform well on both asynchronous and synchronous machines alike.
While the conventional sensorless rotor angular position estimation techniques described above can provide a high fidelity estimate of the rotor position, there are some drawbacks. One such drawback relates to the increase in switching losses incurred in the semiconductor devices due to the introduction of test vectors injected within each PWM cycle or period. In general, prior conventional sensorless rotor angular position estimation techniques based upon PWM test pulse excitation double the switching losses compared to traditional SVPWM.
It is desirable to provide improved methods, systems and apparatus for sensorless rotor angular position estimation. For instance, it would be desirable to provide methods, systems and apparatus for sensorless rotor angular position estimation with reduced switching losses in the inverter module. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.